1. Field of the Invention
The present invention relates to a deposited film forming process, a deposited film forming apparatus, and a process for manufacturing a semiconductor element, and more particularly to a process for manufacturing a photoelectric conversion element with a semiconductor layer comprising microcrystals.
2. Related Background Art
As photoelectric conversion elements such as a photovoltaic element, sensor, or the like, there are known those in which a back face reflective layer made of a material such as ZnO or Ag is formed on a stainless steel substrate; a non-monocrystalline semiconductor film such as an amorphous silicon film having the pin or nip junction is formed thereon; and a transparent electrode of a material represented by ITO or SnO.sub.2 is stacked thereon.
It is important for these photoelectric conversion elements comprised of a non-monocrystalline semiconductor to improve the photoelectric conversion efficiency. In a conventional amorphous silicon photoelectric conversion element, a high interface resistance between a light incident side electrode and a semiconductor layer of a specific conductivity type (p- or n-type semiconductor layer) hindered an improvement in the fill factor (F.F.), so that a significant improvement in the photoelectric conversion efficiency (Eff.) could not be attained. Thus, a microcrystalline semiconductor is employed to reduce the interface resistance between the light incident side electrode and the specific conductivity type semiconductor layer and an improvement in F.F. is achieved by the reduction of resistance due to the microcrystallization. Furthermore, the microcrystallization leads to an improvement in the light transmittance.
However, since an amorphous layer is generally formed by glow discharge decomposition of a mixed gas of SiH.sub.4, H.sub.2 and so on, and since in forming a specific conductivity type semiconductor layer comprised of microcrystals, the microcrystallization of silicon advances with a greater-high frequency power applied to glow discharge electrodes, the microcrystalline layer is formed with the high frequency power being more than several times greater than that when forming amorphous silicon. For this reason, there is a problem that when forming a microcrystalline layer, the i-type semiconductor layer surface, i.e., an interface between the i-type semiconductor layer and a p- or n-type semiconductor layer, is subjected to damage due to the collision of high-speed charged particles of the plasma generated by the glow discharge, whereby the junction of the i-type semiconductor layer and the p-or n-type semiconductor layer become imperfect, interface states increase and the photoelectric conversion efficiency decreases.
Thus, to solve this problem, Japanese Patent Application Laid-Open No. 62-209871 discloses a process of successively increasing the degree of microcrystallization of the i-type semiconductor layer toward the specific conductivity type semiconductor layer. This process includes a way to change the high frequency power or a way to change to the flow rate of H.sub.2 for the successive change in the degree of microcrystallization as mentioned above. However, in the case of using a film forming chamber with a longitudinal discharge chamber as shown in FIG. 3 and continuously carrying a belt-shaped substrate as shown in FIG. 4 to form a semiconductor layer, the implementation of this process is difficult.
Therefore, it is considered to make a part of the i-type semiconductor layer of a microcrystalline layer. For microcrystallization, an increase in high frequency power and a rise in a H.sub.2 dilution ratio may be employed. However, a rise in the H.sub.2 dilution ratio leads to such considerably small film forming rates as 0.1-5 .ANG./sec, so that no microcrystalline i-type layer of a sufficient thickness can be obtained in a discharge chamber with a small length and it requires a long time to form a microcrystalline i-type layer of a sufficient thickness. This presents a problem for mass production.
When a way to reduce the H.sub.2 dilution ratio or to increase a high frequency power is employed for increasing the film forming rate, the degree of microcrystallization in the outermost surface of a microcrystalline i-type semiconductor layer will decrease, thus increasing interface states at an interface with a specific conductivity type semiconductor layer formed on the i-type semiconductor layer (hereinafter, referred to as second conductivity type semiconductor layer). This poses a problem also where the second conductivity type semiconductor layer is amorphous.
Besides, when the second conductivity type semiconductor layer is formed of microcrystalline silicon, a high degree of microcrystallinity in the surface of the i-type layer and a low degree of microcrystallinity of the second conductivity type semiconductor layer stacked thereon would lead to an increase in the interface states of the p/i interface, thereby hindering an improvement in the photoelectric conversion efficiency. For increasing the degree of microcrystallization, an increase in high frequency power, a rise in a H.sub.2 dilution ratio or the like can be adopted, but the film forming rate becomes small and accordingly a long time is necessary for the formation of a sufficiently thick second conductivity type semiconductor layer, thus requiring a very long discharge chamber for obtaining a sufficiently thick second conductivity type semiconductor layer. This has also presented a critical problem for the mass production of photovoltaic elements, as is the case with the microcrystalline i-type semiconductor layer as described above.